1. Field of the Invention
The present invention relates to a fly-back transformer for generating D.C. voltages to be applied to respective electrodes of a Braun tube (or in other words a cathode-ray tube and herein after referred to as a CRT).
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional fly-back transformer. In the figure, reference numeral 1 represents an iron core of the fly-back transformer, and 2 is a primary coil wound on the iron core 1. Reference signs 3a to 3f represent secondary coils wound on the iron core 1, and 4a to 4f are rectifying diodes connected in series with the secondary coils 3a to 3f respectively. Reference numeral 5 represents a resistor connected to a connecting point between the secondary coil 3c and the diode 4d, and 6 is a variable resistor connected to another fixed terminal of the resistor 5. Reference numeral 7 represents a resistor connected in series with the variable resistor 6 at the another fixed terminal thereof, and 8 is a resistor connected to a movable terminal of the variable resistor 6. Reference numeral 9 is a CRT having terminals to which voltages are applied from the fly-back transformer , and 10 is an anode terminal to which an anode voltage is applied from the above-mentioned diode 4a. Reference numeral 11 is a focus terminal to which a focus voltage is applied from the above-mentioned resistor 8.
Next, the operation of the circuit shown in FIG. 1 will be described. At the primary coil 2 of the fly-back transformer, a pulse voltage, which is generated during a fly-back period of a horizontal sweep signal supplied to a horizontal deflection means (omitted in the figure) of the CRT 9, is inputted. Then, the pulse voltage is boosted and induced at the secondary coils 3a to 3f. The induced voltages are rectified and superimposed by the diodes 4a to 4f to obtain a high D.C. voltage. This high D.C. voltage is applied, as an anode voltage, to the anode terminal 10 of the CRT 9 through the diode 4a.
On the other hand, a D.C. voltage is taken out from a part of the secondary coils 3a to 3f (in the illustrated example, from a connecting point between the secondary coil 3c and the diode 4d), and the taken out D.C. voltage is divided by the resistor 5, the variable resistor 6, and the resistor 7 to be a focus voltage which is then applied to the focus terminal 11 of the CRT 9 through the resistor 8 connected to the movable terminal of the variable resistor 6. Here, instead of taking out the D.C. voltage from a part of the secondary coils 3a to 3f, the D.C. voltage may be taken out from a cathode side of any one of the diodes 4a to 4f depending on the required voltage as the focus voltage of the CRT 9.
As documents disclosing arts relating to the above-mentioned conventional fly-back transformer, there are known, for example, Japanese Patent Publication (Kokai) No. 62-136970 and Japanese Patent Publication No. 1-125808.
Since the conventional fly-back transformer is constructed as above, the focus voltage applied to the focus terminal 11 of the CRT 9 is fixed to a constant D.C. voltage. Therefore, in the CRT 9, an electron gun is designed on the condition that such a constant focus voltage is applied. Accordingly, there is a problem in the conventional fly-back transformer in that a part of the focus performance on the whole surface of the CRT 9 is sacrificed.